Piezoelectric transducers have conventionally been used to convert electric signals into sound waves or other mechanical vibrations or to convert mechanical vibrations into electric signals. They convert electric signals into mechanical vibrations by using the morphological change of a crystal by voltage application. Conversely they can use the voltage generated by a pressure applied on a crystal to determine the amount of the pressure.
One application of a piezoelectric transducer is as a probe which is well known for use in an ultrasonic diagnostic equipment for medical purposes or in a nondestructive test unit for materials. For instance, the scanning method of ultrasonic beams, the principle of linear electronic scanning, sector electronic scanning, and the principle of beam deflection are described in a paper entitled "Recent progress in ultrasonic diagnostic apparatuses"; the Journal of Acoustic Society of Japan, Vol. 36, No. 11, 1980, pp. 576-580. The paper also explains how to obtain ultrasonic images for medical uses.
However, the resolution of piezoelectric transducers currently used as a probe is not yet quite satisfactory.
In order to enhance the image resolution in a diagnostic apparatus, it is necessary to improve the positional precision, the time-resolution, and matching in acoustic impedance with a sample.
In order to improve the positional precision, it is desirable to converge ultrasonic beams at a point. The probe which has been used in the linear scanning method of the prior art was defective in that it linearly focuses ultrasonic beams. The sound source should preferably be a curved surface, or more particularly a spherical surface, in order to focus ultrasonic beams at a point.
This applicant has already filed a patent application for a piezoelectric transducer having a curved sound source. (JPA laid-open Sho 60-111600 which is hereinafter referred to as the first application). An embodiment wherein piezoelectric transducer elements having curved surfaces are formed on a curved base is described in the specification and drawings of the first application, and convergence and radiation of acoustic waves are explained. However, the piezoelectric transducer according to this application was not intended to be used as a probe and therefore the invention did not consider the control of beam focus point.
The convergence point of radiated beams could be controlled by the piezoelectric transducer disclosed in the first application if plural piezoelectric transducer elements are formed as concentric annular electrodes, and driving pulses applied to each of the electrodes are staggered timewise. However, the invention mentioned above is still defective because of the following point in time resolution.
In order to improve time-resolution, the reverberation of received waves should be reduced and the time required for damping should be shortened. However if plural electrodes are provided on a dense piezoelectric material, the effect of driving an electrode, especially with a vibration or electric field, would be propagated to other electrodes. A probe emits acoustic waves excited by electric driving pulses toward a target (e.g. the living body), receives the acoustic waves reflected therefrom, and converts them into electric signals again, using a single device for all the above actions. Therefore, if vibration or voltage leaks to other elements, the state is the same as if ultrasonic signals are inputted from outside and this can cause noise and inaccuracy.
As a means to solve the problem, it is proposed to divide the piezoelectric material in addition to the electrodes. The present applicants have filed a patent application for a piezoelectric transducer wherein both piezoelectric material and electrodes are divided and arranged concentrically to improve positional precision as well as time resolution. (Inventors Hikita et al., U.S. Ser. No. 07/487,896 filed on Mar. 6, 1989. Hereinafter referred to as the second application). However, this application did not consider the matching of acoustic impedance.
When mismatching exists in acoustic impedance between the piezoelectric material and a living body or water, the sound generated from the piezoelectric transducer is greatly damped when reflected from a target. When the amount of damping is large, the sensitivity in received signals deteriorates, presenting a difficulty in obtaining clear images. Therefore, the acoustic impedance of a piezoelectric transducer should preferably be close to that of the water when used as a probe in an ultrasonic diagnostic apparatus.
This invention was conceived to solve the above mentioned problems in the prior art, and aims to provide a piezoelectric transducer which can prevent deterioration of resolution which would otherwise be caused by noise or reverberations due to transmission of vibrations between adjacent piezoelectric transducer elements and which has an acoustic impedance closer to that of water.